I just ran into this and the problem disappeared as soon as I determined enough height for the content. In Xcode, it might still look fine, but AutoLayout decided to dispense with the TextView if it didn't have a height limit.
It was probably not your problem when you asked this question, but it still appeared quite early in my Google search, so I am posting this answer anyway.
Btw: Xcode is still a bit valid when you edit the constraint. It will update the view (and save) if you press 'Enter' in the Constant-Field, but it will not do it if it loses focus in some other way.
This is just to show us how difficult it is to constantly access user interfaces.
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